Conventionally, integrated circuits (IC), or large-scale integrated circuits (LSI) have been connected to a printed substrate by soldering process. However, soldering has failed to connect the IC or the LSI to the printed substrate efficiently. Furthermore, it has been hard to improve the packaging density of the IC or the LSI by using soldering.
In order to solve these problems, there has been developed a BGA (ball grid array) package, in which the electronic devices are connected to a substrate by using a so-called solder ball. In a BGA, heating causes the solder balls to melt, and connection between the device and the printed circuit board takes place. As the result, not only the production efficiency but also the packaging density can be greatly improved by using the BGA package.
However, since multi-layering of a substrate has been developed in recent years and a multilayer substrate is vulnerable to an effect of usage environment, there has been a problem that disconnection tends to occur due to the stress and strain appearing in the substrates.
For example, in a case where a semiconductor is connected to a substrate by using a solder ball, a stress is applied to the solder ball because the linear coefficient of expansion is different between the semiconductor and the substrate. As a result, there has been a case where a crack occurs in the solder ball and disconnection is caused.
In response to the above problem, Patent Document 1 discloses a conductive fine particle which comprises a metal layer containing a high-conductive metal, which is formed on the surface of a resin fine particle, and a solder layer further formed on the surface of the metal layer. When a conductive fine particle of this kind is used, the flexible resin particle can alleviate the stress applied to the conductive fine particle. Since a solder layer is formed on the outermost surface of the conductive fine particle, conductive connection between electrodes is easily allowed.
However, when the conductive fine particle, in which a solder layer is formed on the surface of a resin fine particle, is used in electronic device such as a cell-phone, there has been an issue where a drop impact and the like causes a crack in the solder layer or breakage in a connection interface between the electrode and the conductive fine particle. There has been a problem that the crack in the solder layer and breakage in the connection interface causes disconnection between the electrode and the conductive fine particle.
Further, in a case where a conductive fine particle with a solder layer formed therein is mounted onto the electrodes of substrates, the conductive fine particle is placed on an electrode formed on a first substrate and the solder layer is melted by reflowing, so that the conductive fine particle is fixed to the first electrode (process 1). Subsequently, an electrode formed on a second substrate and the electrode formed on the first substrate are placed opposite to each other and then reflowing is carried out, so that conductive connection is allowed between the electrodes on the substrates (process 2).
However, when the electrodes on the substrates are conductively connected with use of the conductive fine particle disclosed in Patent Document 1, there has been a case where a solder layer is melted in the process 1 and the solder layer adequate for conductively connecting the electrode on the second substrate is not left on the surface of the metal layer of the conductive fine particle. As a result, there has been a case where conductive connection is not allowed.
Further, a temperature in an electronic device rises because of heating in an electronic part of the electronic device during use and the temperature in the electronic device is lowered to room temperature after the use of the electronic device. Accordingly, repetition of heating and cooling, namely, “heat cycle” is conducted in the electronic device. Repetition of this heat cycle may have caused a case where thermal fatigue occurs in the solder layer and the connection interface of the electrode and the conductive fine particle is destroyed, so that disconnection is caused.    Patent Document 1: Japanese Kokai Publication 2001-220691 (JP-A 2001220691)